Several decades of intensive laboratory investigations and clinical studies revealed that combinations of radiation and chemotherapy can yield a moderate improvement in local-regional control and survival rates of a number of cancers but too often at the expense of increased toxicity. This observation underscores the need for refinement of combined therapy strategies to further improve the therapeutic index. Advances in the understanding of tumor biology, cellular and molecular response to radiation, cytotoxic and biologic agents, and radiation-drug interactions open new opportunities to design combined regimens based on specific mechanisms of interaction or molecular targets to selectively enhance tumor response. Recent studies showed that many neoplasms have a high magnitude of epidermal growth factor receptor (EGFR) or cyclo-oxygenase-2enzyme (COX-2) expression. Experimental data from our laboratory and other groups demonstrated that EGFR overexpression is associated with resistance to treatment. We have generated compelling preliminary data showing that blockade of the EGFR pathway or that inhibition of COX-2 lead to marked sensitization of tumor to radiotherapy. The long-term goal of this project is to design combined therapy regimens targeting molecular pathways that are differentially expressed between tumors and normal tissue to maximize therapeutic gain, i.e., enhancement of tumor response with no or less increase in morbidity. The specific aims for this proposed project are to: 1) investigate the mechanisms of radiation-C225 interaction in vivo; 2) optimize the combination of fractionated radiotherapy with C225 using human xenograft models; 3) develop rational triple-therapy strategies consisting of the combination of radiotherapy with a classical cytotoxic agent and C225 in xenograft models for clinical testing; the most promising preclinical regimen will undergo phase I clinical testing in patients with advanced HNSCC; and 4) assess the combined effects of radiation with biologic therapy aiming simultaneously at two molecular targets, i.e., EGFR blockade and COX-2 inhibition, each of which sensitizes tumors to radiation.